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Which step in the decay scheme explains how \[{}^{210}Pb\] ends up in rain water while its parent ${}^{238}U$ is only present in earth crust?
$A.$ ${}^{238}U - {}^{234}U$
$B.$ ${}^{234}U - {}^{230}Th$
$C.$ ${}^{230}Th - {}^{226}Ra$
$D.$ ${}^{226}Ra - {}^{222}Rn$

Last updated date: 13th Jun 2024
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HINT- The decay scheme of radioactive substances is a graphical representation of all the transitions occurring in a decay, and of their relationships. It is useful to think of the decay scheme as placed in a coordinate system, where the ordinate axis is energy, increasing from bottom to top, and the abscissa (x-axis) is the proton number, increasing from left to right. The arrows indicate the emitted particle. For the gamma rays, the gamma energies are given; for the beta decay the maximum beta energy.

Complete step by step solution:
Now moving on to the question, Radium 226 produces Radon 222 on decaying. Radon gas can escape to the atmosphere before it decays and after a residence time, it decays to polonium 218 which falls to the surface of the earth with dust and rain. Eventually, Lead 210 produced and thus can end up in rainwater.

This gives option $D.$${}^{226}Ra - {}^{222}Rn$ as the correct option.

NOTE- Uranium 238, the most prevalent isotope in uranium ore, has a half-life of about 4.5 billion years; that is, half the atoms in any sample will decay in that amount of time. Uranium 238 emits alpha particles which are less penetrating than other forms of radiation, and weak gamma rays. As long as it remains outside the body, uranium poses little health hazard (mainly from the gamma rays). If inhaled or ingested, however, its radioactivity poses increased risks of lung cancer and bone cancer. Uranium is also toxic at high concentrations and can cause damage to internal organs, notably the kidneys.
The property of uranium important for nuclear weapons and nuclear power is its ability to fission, or split into two lighter fragments when bombarded with neutrons releasing energy in the process.